1. Field of the Invention
This invention relates generally to methods and apparatus for depositing plating materials on a substrate, and more particularly, to a method and apparatus for employing a railgun to perform plating and/or material build up.
2. Description of the Related Art
Electrolytic plating is a process of building up layers of material on the surface of a substrate. The electrolytic plating process has some limitations. For example, electrolytic plating is inconvenient when employed for large objects, it has a limited ability to form layers having an inhomogeneous thickness, and some materials are difficult to use because of the chemical properties of the materials. The prior art has developed thermal spray and explosive techniques for depositing material on a substrate, in part, to avoid the above-mentioned problems.
The thermal spray techniques employ the thermal expansion of a gas to power the deposition of the plating material. Examples of thermal spray techniques include flame, high velocity oxygen fueled (HVOF), detonation-gun (D-Gun), and plasma spraying. Thermal spray techniques can impart velocities to the plating material not exceeding the velocity of the expanding gases that power the underlying thermal process. Thermal velocities of gases are limited to about 1 kilometer (km) per second. Since plating particles having velocities of 1 km per second ordinarily have kinetic energies lower than the heat of fusion of the particles, thermal spray techniques do not ordinarily melt plating particles upon impact with the substrate. Thermal spray techniques do not ordinarily enable forming the strong bond between the plating material and the substrate that would result if the plating particles melted upon impact.
The prior art has employed explosive deposition processes in situations where it is desirable to directly deposit initially solid plating materials. To use explosive processes, the plating material is formed into a shell. For example, the plating material may be a shell of aluminum foil. Then, an explosion, e.g., an oxyacetylene explosion, catpults the shell of plating material towards the substrate to be plated.
Explosive deposition techniques also have limitations. First, the deposited plating layer ordinarily contains both the plating material and the shell matrix, because both the shell and the plating material are catapulted towards the substrate by the explosion. To eliminate plating impurities, the shell matrix can be fabricated with the plating material itself. But, expensive plating materials lead to prohibitive costs when used to construct the shell matrix. Second, explosive techniques are not amenable to the high repetition rates that are necessary to deposit thick plating layers in a controlled fashion. The explosive technique does not offer a versatile method for building up layers of material on a substrate. Third, explosive techniques do not accelerate the plating materials to velocities greater than velocities of the order of the speed of sound. Fourth, the technique is not amenable to complex geometric surfaces. Fifth explosive/thermal spray techniques tend to produce plating deposits with a substantial number of voids. The voids can result in the development of cracks in plating layers that are thicker than about 0.030 inches.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.